The present invention relates generally to an electronic control system for an internal combustion engine for controlling fuel injection, spark ignition timing, air/fuel mixture ratio, exhaust gas recirculation rate and so on. More particularly, the invention relates the electronic engine control system having a pressure sensor which is capable of selectively introducing an intake vacuum pressure and atmospheric air pressure as one of the engine control parameters.
Recently, electronic engine control systems have become popular in the automotive vehicle field. In such a control system, engine operation is optimized by adapting to changing operating conditions. For example, the fuel amount to be supplied to the engine cylinders is controlled generally based on the engine speed and engine load in order to adapt to changing engine operating conditions. For determining the load condition on the engine, an intake vacuum sensor for determining the vacuum pressure in the intake manifold or an air flow meter for determining the intake air flow rate are commonly used. The determined engine load is preferably corrected for variations in atmospheric air pressure. Conventionally, the intake vacuum pressure and the atmospheric air pressure are determined separately by respective pressure sensors. This has increased the cost of the control system.
A relatively low-cost control system has been proposed in Tokkai Sho No. 54-153929 published on Dec. 4, 1979 in Japan. In this prior art, the initial preoperation pressure determined by the intake vacuum sensor, which corresponds to the atmospheric air pressure, is used as a reference value for correcting the determined engine load. However, such a system still has the drawback that it cannot follow variations of the ambient or atmospheric air pressure during engine operation.